Inverted Light-Emitting Diode Having Plasmonically Enhanced Emission

ABSTRACT

An LED device having plasmonically enhanced emission is provided. The device includes an inverted LED structure with a coating of metal nanoparticles on the surface chosen to match the plasmonic response to the peak emission from the active quantum well (QW) emission region of the LED. The active QW emission region is separated from the metal nanoparticles on the surface by a thin n-type contact layer disposed on a top side of the active QW emission. A p-type layer is disposed immediately beneath the active QW emission region and injects holes into the active QW emission region. The n-type contact layer is sufficiently thin to permit a coupling of the surface plasmons (SPs) from the metal nanoparticles and the excitons in the active QW emission region. The SP-exciton coupling provides an alternative decay route for the excitons and thus enhances the photon emission from the LED device.

CROSS-REFERENCE

This application is a Divisional of, and claims the benefit of prioritybased on, U.S. patent application Ser. No. 13/311,673 filed on Dec. 6,2011, which is a Nonprovisional of, and claims the benefit of prioritybased on, U.S. Provisional Patent Application No. 61/424,103 filed onDec. 17, 2010, both of which are hereby incorporated by reference intothe present application in their entirety.

TECHNICAL FIELD

The present invention relates to semiconductor-based devices,specifically light-emitting diodes.

BACKGROUND

The efficiency of semiconductor light-emitting diodes (LEDs) hascontinuously improved since the first fabrication of an infrared devicein 1960. The key metric has been the external quantum efficiency (EQE),which describes the number of photons externally radiated per electroninjected into the device. The external quantum efficiency is the productof the injection efficiency, internal quantum efficiency, and extractionefficiency.

The injection efficiency of an LED describes the fraction of electronsthat are injected into the active region relative to the number ofelectrons flowing into the active region. The internal quantumefficiency describes the ratio of photons generated in the active regionby radiative transitions relative to the sum all radiative andnon-radiative processes, and can be improved by increasing the rate ofradiative transitions or by decreasing the rate of non-radiativetransitions, e.g., by introducing additional radiative states or statesthat couple to radiative states.

The extraction efficiency of an LED describes the fraction of photonsthat escape the device relative to the number of photons generated inthe active region. When the electron-hole pairs generated in the activeregion of an LED decay, they can either decay radiatively by emitting aphoton and thus producing light, or can decay non-radiatively asdissipated energy in the crystal lattice, for example by producing heat.In a simple planar LED, the light is emitted in an “escape cone” whoseproperties are defined by the LED's angle of total internal reflectionat the semiconductor/atmosphere interface. The remainder of the light istrapped within the LED structure by total internal reflection and canonly escape at the ends of the semiconductor slab making up the LED orthrough some imperfection in the device surface.

A major vein of LED research is directed to enhancing the extractionefficiency of light from the semiconductor into the surroundingatmosphere by modifying the optical structure in or around thesemiconductor host.

Some researchers have employed a microcavity in the LED to guide thepreferential propagation of generated photons into the light extractioncone of the LED. See M. Mastro, J. Caldwell, R. Holm, R. Henry, and C.Eddy Jr., “Design of Gallium Nitride Resonant Cavity Light-EmittingDiodes on Si Substrates,” Adv. Mater. 2008, 20, 115-118.

Others have used a photonic crystal deposited on an LED to diffract andredirect the light out of the semiconductor and into the atmosphere. SeeM. Mastro, C. Kim, M. Kim, J. Caldwell, R. Holm, I. Vurgaftman, J. Kim,C. R. Eddy, Jr., and J. Meyer, “Zinc Sulphide Overlayer Two-DimensionalPhotonic Crystal for Enhanced Extraction of Light from a Micro CavityLight-Emitting Diode” Japanese Journal of Applied Physics, Vol. 47, No.10, 2008, pp. 7827-7830.

Still others have attempted to improve the extraction of light from anLED by roughening a surface of the semiconductor structure, thusscattering the emitted photons and enhancing the light extractionefficiency. See B. Kim, M. Mastro, H. Jung, H. Kim, S. Kim, R. Holm, J.Hite, C. Eddy Jr., J. Bang, and J. Kim, “Inductively coupled plasmaetching of nano-patterned sapphire for flip-chip GaN light emittingdiode applications” Thin Solid Films 516 (2008) 7744-7747; and B. J.Kim, H. Jung, J. Shin, M. Mastro, C. Eddy Jr., J. Hite, S. Kim, J. Bang,and J. Kim, “Enhancement of light extraction efficiency of ultravioletlight emitting diodes by patterning of SiO₂ nanosphere arrays, ThinSolid Films 517 (2009) 2742-2744.

Another approach to improving light extraction from an LED has been toinvert the structure of the LED itself. In a traditional GaN-based LEDstructure, the final grown layer is a p-type GaN contact layer to avoidmagnesium memory effects and doping-generated growth defects. Use oflow-resistance Ohmic contacts to the p-type region requires a high levelof magnesium doping in the p-type contact layer. To minimize the growthof and contacting to the p+GaN contact layer, an inverted LED structurehas been suggested by various commercial entities. See, e.g., U.S. Pat.No. 7,170,097 to Edmond et al., entitled “Inverted Light Emitting Diodeon Conductive Substrate”; and T. Takeuchi, G. Hasnain, S. Corzine, M.Hueschen, R. Schneider, Jr., C. Kocot, M. Blomqvist, Y. Chang, D.Lefforge, M. frames, L. Cook, and S. Stockman, “GaN-Based Light EmittingDiodes with Tunnel Junctions,” Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L861-L 863.

Operation of such an inverted LED device is similar to operation of astandard LED in that electron and holes from n- and p-type regions,respectively, are injected into a quantum well active region. However,in most cases an inverted LED structure has both Ohmic contacts made ton-type material. This creates an npn semiconductor that would prohibitcurrent flow in either the forward or reverse direction fornon-degenerate doping levels. Such an inverted LED, biased by two n-typecontacts, will often include a (degenerate) n++/p++ tunnel junction thatunder local reverse bias allows electrons to quantum-mechanically tunnelthrough a thin depletion region into the p-type layer adjacent to theactive region. At this point, the operation is similar to a traditionalLED where holes are injected into the active region from the adjacentp-type layer and electrons are injected from the adjacent n-type layerinto the active region, where the electrons and holes may recombineradiatively.

Still another approach has found that introduction of a plasmonic metalin the near vicinity (tens of nanometers) of the semiconductor activeregion can improve the emission of light from an LED.

A surface plasmon (SP) from such plasmonic metals at a perfectly flatmetal/semiconductor interface is in the form of a non-propagatingevanescent wave. If the SP is not coupled to the excitons (i.e., theelectron-hole pairs) formed in the active quantum well (QW) region, anyemitted photon can be non-radiatively absorbed into the metal surfaceand dissipated as heat. See C. Langhammer, Z. Yuan, I. Zorić, and B.Kasemo, “Plasmonic Properties of Supported Pt and Pd Nanostructures,”Nano Letters 2006 Vol. 6, No. 4 833-838. However, when the excitonswithin the active QW region and the SPs within the plasmonic metal arecoupled, the energy in the exciton transitions into the surface plasmonstate in the metal or at the metal/dielectric (which is usually asemiconductor) interface. The surface plasmon will oscillate orpropagate for a short time but eventually either will dissipate as heatvia a phonon or will scatter as a photon in the air and be emitted fromthe device. Thus, by coupling the SPs and the excitons, an alternativedecay route for the excitons is created, which improves the probabilitythat the exciton will decay into an SP and scatter, thus produce light.K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen,Li Wang, F. Y. Jiang, X. W. Sun and H. D. Sun, Appl. Phys. Lett., 94,151102 (2009). This coupling occurs when the energy of the electron-holepair has sufficient overlap in energy with the energy of the surfaceplasmon (SP) resonance condition.

The QW must be in close proximity to the plasmonic metal in order forthe QW excitons to be located within the SP fringing field so that theymay be coupled into the SP state. Okamoto et al. calculated the fringingfield depth as 47 and 77 nm for silver and aluminum, respectively. SeeK. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A.Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantumwells,” Nature Materials, Vol. 3, No. 9, pp. 601-605 (2004).

Many approaches have been taken to improve the SP-exciton coupling andenhance the emission of scattered light. For example, Okamoto et al.found large enhancements in the emission from InGaN quantum well (QW)devices when a thin film of platinum or silver was deposited 10 nm abovethe QW, relying on imperfections and roughness in the metal coating tocouple the surface plasmons into scattered light. See Okamoto, supra.

Another approach involves structuring the plasmonic metal asnanoparticles. Such structuring allows for tuning of the SP resonancecondition where the SP density of states (DOS) is extremely high, toclosely match the peak emission of the active layer of the semiconductoremitter. In addition, the periodicity of the metal nanoparticles allowsa more controlled scattering of the SPs into radiative photons. See A.Neogi and H. Morkoç, “Resonant surface plasmon-induced modification ofphotoluminescence from GaN/AlN quantum dots,” Nanotechnology 15 (2004)1252-4255, who deposited arrays of silver nanoparticles onto a structurewith GaN quantum dots located in close proximity to the surface.

However, as noted above, in order to achieve coupling of the exciton andthe SP, the plasmonic metal must be situated very close, usually within30 nm, to the active QW of the LED device. This necessity presents asubtle dilemma to construction of a functional plasmonic enhanced(In,Al)GaN LED. In a traditional GaN-based LED structure, use oflow-resistance Ohmic contacts to the p-type region is based on magnesiumdoping in the p-type contact layer. However, -magnesium is a deepacceptor and cannot produce a high level of activated holes in(In,Al)GaN. The low hole concentration in the traditional p-type GaN topcontact layer requires a thickness of at least 200 nm to achieve propercurrent spreading. In addition, a thin, i.e., less than 80 nm, p-type(In,Al)GaN top contact layer would exhibit band bending from the surfacestates that would hamper the injection of holes into the active region.

The magnesium dopant also has a high activation energy, which limits thedensity of active acceptors to approximately 10¹⁸ cm⁻³ at roomtemperature. For a simple pn junction, this corresponds to a p sidedepletion width of 32.2 nm and an n side (with 5×10¹⁸ cm⁻³ activedonors) depletion width of 6.5 nm. Once depletion from the surface isincluded, the necessary theoretical thickness of the p-type region isalready beyond the SP fringing field depth and thus coupling will notoccur. In practice, the high level of defects also results in lowcarrier mobility and thus high resistivity of the p-GaN, whichnecessitates a thick (100 to 250 nm) current-spreading p-type layer incommercial LEDs, which only exacerbates this issue.

Kwon et al. attempted to avoid this issue by interrupting the growth,placing the silver nanoparticles internal to the semiconductor, and thencontinuing the growth; specifically, the plasmonic metal layer wassituated in the n-GaN within a few nanometers of the multi-QW. See M.Kwon, J. Kim, B. Kim, I. Park, C. Cho, C. Byeon, and S. Park,“Surface-Plasmon-Enhanced Light-Emitting Diodes” Adv. Mater. 2008, 20,1253-1257. Although Kwon et al. maintained that the semiconductorcrystal was not perturbed by the discontinuous silver interlayer, anyforeign particle would very likely disturb the crystallographic stackingduring epitaxy, and SPs of the silver interlayer that do scatter areemitted as photons within the semiconductor slab and so will suffer fromextraction issues (e.g., total internal reflection) that are problematicfor all LEDs.

Thus, it would be advantageous to place the plasmonic metal exterior tothe semiconductor slab making up the LED so that the SP-exciton energycouples from the active region to the metal, thus avoiding the necessityto exit via the semiconductor/air light cone.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

The present invention provides an LED device having plasmonicallyenhanced emission.

A device in accordance with the present invention includes an invertedLED structure with a coating of metal nanoparticles on the surface. Thecoating would be in the form of a periodic or random coating of silver,gold, copper or similar plasmonic metal chosen to match the plasmonicresponse to the peak emission from the active quantum well (QW) emissionregion of the LED.

The active QW emission region is separated from the metal nanoparticleson the surface by a thin n-type contact layer disposed on a top side ofthe active QW emission region which injects electrons into the active QWemission region and is contacted to an n-type Ohmic electrode. Then-type layer is less than 50 nm thick, and preferably is less than 30 nmthick.

A p-type layer that injects holes into the active QW emission region isdisposed immediately beneath the active QW emission region such that theactive QW emission region is between the n-type layer and the p-typelayer. In some embodiments the p-type layer is a relatively thick p-typelayer contacted to a p-type Ohmic electrode and deposited on a substratesuch as sapphire, SiC, Si, GaN, ZnO or AN that is not involved in thecurrent transport. In other embodiments, the p-type layer is arelatively thin p-type layer contacted by a p-type electrode and grownon a p-type substrate such as p-type SiC or p-type GaN.

In still other embodiments, both the top and bottom electrodes aren-type Ohmic contacts. In such embodiments, one n-type Ohmic contact ismade to the n-type contact layer at the top of the LED structure. Theother n-type Ohmic contact is made to an n-type GaN base layer at thebottom of the LED structure or to an n-type substrate such as SiC withan overlying n-type GaN base layer, with an n⁺⁺/p⁺⁺ tunnel junctiondisposed onto the n-type GaN base layer. Current will quantummechanically tunnel from the n-type GaN layer through the tunneljunction into a p-type GaN layer from which holes will be injected intothe active quantum well emission region.

In all cases, the n-type contact layer is sufficiently thin to permit acoupling of the surface plasmons (SPs) from the metal nanoparticles andthe excitons, i.e., the electron-hole pairs, formed in the active QWemission region. The SP-exciton coupling provides an alternative decayroute for the excitons and thus enhances the photonic emission from theLED device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a first exemplary embodiment of an inverted LED havingplasmonically enhanced emission in accordance with the presentinvention.

FIG. 2 depicts a second exemplary embodiment of an inverted LED havingplasmonically enhanced emission in accordance with the presentinvention.

FIG. 3 is a scanning electron micrograph of silver nanoparticlesdeposited on a surface of an inverted LED having plasmonically enhancedemission in accordance with the present invention.

FIG. 4 is a plot of a simulation illustrating the extinction efficiencyfor an array of silver nanoparticles situated at an Al_(x)Ga_(1-x)N/airinterface.

FIGS. 5A and 5B are plots illustrating the photoluminescence acquired attwo different spectral ranges of an inverted LED having plasmonicallyenhanced emission in accordance with the present invention, where FIG.4A illustrates photoluminescence in the range of about 350-600 nm andFIG. 4B illustrates photoluminescence in the range of about 350-390 nm.

DETAILED DESCRIPTION

The aspects and features of the present invention summarized above canbe embodied in various forms. The following description shows, by way ofillustration, combinations and configurations in which the aspects andfeatures can be put into practice. It is understood that the describedaspects, features, and/or embodiments are merely examples, and that oneskilled in the art may utilize other aspects, features, and/orembodiments or make structural and functional modifications withoutdeparting from the scope of the present disclosure.

For example, although the present invention may be described in thecontext of a device composed of n-type (In,Al)GaN and p-type GaN layers,the plasmonically enhanced light emitting device in accordance with thepresent invention can be composed of any semiconductor materials,including GaN/AlGaN/InGaN/InAlGaN, ZnO/MgZnO, ZnSe/ZnMgSe, CdTe/CdZnTeand similar semiconductors, and all such devices are contemplated to bewithin the scope of the present invention. The present invention is mostadvantageous to semiconductor materials that are difficult to producewith high-quality n-type and p-type regions. The quality can be definedas crystal quality. The quality can also be defined as the ability toachieve both high electron and hole carrier concentrations, which isparticularly problematic in wide-bandgap semiconductors. See M. Mastro,“Fundamentals and future of semiconductor device technology,” in “III-VCompound Semiconductors: Integration with Silicon-BasedMicroelectronics”, CRC/Taylor & Francis, 2011.

The present invention provides an LED device having plasmonicallyenhanced emission.

An inverted LED in accordance with the present invention includes anystructure that has a thin n-type region between the active region andthe surface of the semiconductor. One inverted or reversed structure isan (Al,In,Ga)N LED in which the n-type layers are above the activeregion and the p-type layers are below the active region. This n-typeregion/active region/p-type region is the reverse of the standard(Al,In,Ga)N LED growth stack of p-type region/active region/n-typeregion/substrate.

In the (Al,In,Ga)N layer or layers, a high n-type carrier concentrationis easily attainable by doping with silicon. The high doping allows thethin n-type layer between the surface of the semiconductor and theactive region to properly spread the current from the contact.Additionally, the top thin layer requires a high doping to prevent thestates at the bare surface or metallized surface from interacting withband structure at the active region.

Thus, as described in more detail below, a device in accordance with thepresent invention includes an inverted LED structure with a coating ofmetal nanoparticles on the surface. The coating would be in the form ofa periodic or random coating of silver, gold, copper or similarplasmonic metal chosen to match the plasmonic response to the peakemission from the active quantum well (QW) emission region of the LED.

The active QW emission region is separated from the metal nanoparticleson the surface by a thin n-type (In,Al)GaN contact layer disposed on atop side of the active QW emission region which injects electrons intothe active QW emission region and is contacted to an n-type Ohmicelectrode. The n-type layer is less than 50 nm thick, and preferably isless than 30 nm thick.

A p-type layer that injects holes into the active QW emission region isdisposed immediately beneath the active QW emission region such that theactive QW emission region is between the n-type layer and the p-typelayer. In some embodiments the p-type GaN layer is a relatively thick,typically about 500 to 5000 nm, p-type layer contacted to a p-type Ohmicelectrode and deposited on a substrate such as sapphire, SiC, Si, GaN,ZnO or AN that is not involved in the current transport. In otherembodiments, the p-type GaN layer is a relatively thin, typically about10 to 500 nm, p-type layer contacted by a p-type electrode and grown ona p-type substrate such as p-type SiC or p-type GaN.

In still other embodiments, both the top and bottom electrode are n-typeOhmic contacts. In such embodiments, one n-type Ohmic contact is made tothe n-type contact layer at the top of the LED structure. The othern-type Ohmic contact is made to an n-type GaN base layer at the bottomof the LED structure or to an n-type substrate such as SiC with anoverlying n-type GaN base layer, with an n⁺⁺/p⁺⁺ tunnel junctiondisposed onto the n-type GaN base layer. Current will quantummechanically tunnel from the n-type GaN base layer through the tunneljunction into a p-type GaN layer from which holes will be injected intothe active quantum well emission region.

In all cases, the n-type contact layer must be sufficiently thin so thatthe plasmonic metal is close enough to the active QW region to permit acoupling of the surface plasmons (SPs) from the metal nanoparticles andthe excitons, i.e., the electron-hole pairs, formed in the active QWemission region. When the excitons within the active QW region arecoupled to the SPs within the plasmonic metal, the energy in the excitontransitions into the surface plasmon state in the metal or at themetal/dielectric (which is usually a semiconductor) interface. Anynon-uniformity in the metal will introduce a scattering site that willbridge the momentum mismatch between the plasmon and the photon. Thesurface plasmon thus will oscillate or propagate for a short time buteventually will dissipate as heat via a phonon or alternatively willscatter as a photon in the air and be emitted from the device. TheSP-exciton coupling thereby provides a decay route for the excitons inthe active QW region in addition to the decay route taken by non-coupledexcitons and thus enhances the photonic emission from the LED device.The decay route for the coupled plasmon resulting in a photon occurswhen the resonance of the plasmonic state is matched to the emission ofthe quantum well, which is typically the intense near-bandedge emission,with the peak resonance of the plasmonic states being determined by itscomposition, geometry, and local dielectric environment including thecomposition of the semiconductor.

FIGS. 1 and 2 illustrate two exemplary embodiments of an inverted LEDdevice having plasmonically enhanced emission in accordance with thepresent invention.

As illustrated in FIG. 1, in a first exemplary embodiment of an LEDdevice in accordance with the present invention, a plasmonicmetal-coated inverted LED heterostructure is provided, wherein thestructure includes an active quantum well region such as QW region 101,highly n-type doped n+ AlGaN layer 102 disposed on an upper side of QWregion 101 to inject electrons into QW region 101, and a highly p-typedoped p+ GaN layer 103 disposed on the opposite side of QW region toinject holes. As noted above, the n-type layer in an LED structure inaccordance with the present invention is thin, less than 50 nm thick,preferably less than 30 nm thick, and the p-type layer has a thicknesssufficiently large to spread the current over the LED area, typicallyabout 500 to 5000 nm, and so in the embodiment shown in FIG. 1, n+ AlGaNlayer 102 has a thickness of 20 nm and p+ GaN layer 103 has a thicknessof 2000 nm. Active QW region 101 will typically be in the form of alayered (Al)GaN structure, for example, 5 nm AlGaN/4 nm GaN/5 nm AlGaN,though other materials having other thicknesses may be used asappropriate In the embodiment illustrated in FIG. 1, Mg is used as adopant though other dopant materials and doping levels may be used asappropriate.

The n+ AlGaN layer 102 can be directly contacted with n-type Ohmicelectrode 105, while p+ GaN layer 103 can be directly contacted by ap-type Ohmic electrode 106. The p-type layer 103 can be grown on asubstrate 108 such as sapphire that is not directly involved in theelectrical device operation and is separated from the p-type layer by abuffer layer, for example, 25 nm AN buffer layer 107.

In addition, as shown in FIG. 1, in accordance with the presentinvention, a periodic or random coating 104 of silver, gold, silver,copper or similar plasmonic metal is disposed on n+ AlGaN layer 102, themetal being chosen to match the plasmonic response to the peak emissionfrom active QW region 101.

The plasmonic metal or a metal alloy with at least one component being aplasmonic metal can be used as the Ohmic contact to the structure;otherwise a separate Ohmic contact is needed. Current is injected intothe device and excitons are formed in the active QW region in accordancewith standard semiconductor physics principles well known in the art.

Because the n-type contact layer between the active QW region and theplasmonic metal surface is thin enough to cause the excitons to bewithin the SP fringing field, in accordance with the present invention,the excitons within the active QW region can couple to the SPs within orin close vicinity of the plasmonic metal, and as described above, thisSP-exciton coupling thereby provides an alternative decay route for theexcitons and enhances the photonic emission from the LED device.

FIG. 2 illustrates a second exemplary embodiment of an LED device inaccordance with the present invention. An LED device in accordance withthis embodiment avoids the necessity for a thick p-type layer as neededin the first embodiment and can be grown in a single step, thussimplifying fabrication.

This embodiment utilizes a tunnel junction 206 which can be composed of,for example, a 5 nm layer of p++ GaN 206 a disposed on a 5 nm layer ofn++ GaN 206 b which in turn is disposed on an n-type substrate such asn+ 4H—SiC layer 208 having an n+ AlGaN cap 207. The use of such a tunneljunction avoids the necessity for a thick p-type layer as in the firstembodiment, since the p- and n-type doping in tunnel junction 206 issufficiently high to eliminate the p- and n-depletion barriers, therebyenabling the electrons to pass directly through junction 206. This inturn allows a non-rectified current flow from 4H—SiC layer 208 into then+ AlGaN cap layer 207 and thus successful operation of the LED.

Thus, as illustrated in FIG. 2, a second exemplary embodiment of an LEDdevice in accordance with the present invention can consist of an activeregion such as active QW region 201, a highly n-type doped n+ AlGaNlayer 202 disposed on an upper side of QW region 201 to inject electronsinto active QW region 201, and a highly p-type doped p+ GaN layer 203 onthe opposite side of the active QW region 201 to inject holes. As withthe structure in the first exemplary embodiment described above withrespect to FIG. 1, in the embodiment illustrated in FIG. 2, active QWregion 201 will typically be in the form of a layered (Al)GaN structure,for example, 5 nm AlGaN/4 nm GaN/5 nm AlGaN, though other materialshaving other thicknesses may be used as appropriate. Also, as with thefirst embodiment, the n-type layer in the LED structure according tothis embodiment of the present invention is thin, less than 50 nm thick,preferably less than 30 nm thick. However, as noted above, the p-typelayer in this second embodiment can be thinner than as in the firstembodiment, typically about 10 to 1000 nm, due to the presence of tunneljunction 206 since the current is already and easily spread in thehighly conductive n-type layers or substrate. Thus, in the embodimentshown in FIG. 2 as in FIG. 1, n+ AlGaN layer 202 has a thickness of 20nm and p+ GaN layer 203 has a thickness of 500 nm.

As with the first embodiment, a periodic or random coating 204 ofsilver, gold, silver, copper or similar plasmonic metal is disposed onn+ AlGaN layer 202, with the material used being chosen to match theplasmonic response to the peak emission from the active QW region 201.

As with the previous embodiment, in this embodiment of a plasmonicallyenhanced inverted LED in accordance with the present invention then-type contact layer between the active QW region and the plasmonicmetal surface is thin enough to cause the excitons to be within the SPfringing field such that the excitons within the active QW region coupleto the SPs within or near the plasmonic metal, and as described above,this SP-exciton coupling thereby provides an alternative decay route forthe excitons and enhances the photonic emission from the LED device.

The inverted LED structures as depicted in FIGS. 1 and 2 were grown inan impinging-flow metal-organic chemical vapor deposition system. Thestructure shown in FIG. 1 was grown in two steps to avoid the Mg memoryeffect known in the art. Specifically, 2000 nm p-type GaN layer 103 wasgrown on a double sided polished sapphire substrate 108. Followingdeposition of the p-type layer 103, the reaction chamber was cleaned,and subsequently active QW layer 101 and n-type AlGaN contact layer 102were deposited on the p-type layer 103.

In contrast, the structure shown in FIG. 2 can be grown in onecontinuous step. The p- and n-type doping in tunnel junction 206 issufficiently high to eliminate the p- and n-depletion barriers, therebyenabling the electrons to pass directly through the junction and injectcarriers into the active QW region 201. This in turn allows anon-rectified current flow from the 4H—SiC layer 208 into the n+ AlGaNcap layer 207 and thus successful operation of the LED.

FIG. 3 illustrates an exemplary embodiment of the plasmonic metalcoating disposed on an inverted LED structure in accordance with thepresent invention. In such an exemplary embodiment, the plasmonic metalcoating consists of a discontinuous silver film with a target thicknessof approximately 100 nm deposited in an e-beam evaporator onto thesurface of the LED structure. The silver islands were exposed to a rapidthermal anneal, e.g., for 1 min at 700° C. This resulted in the coatingillustrated in FIG. 3, consisting of ensemble of silver nanoparticlesdispersed on the sample surface with an average diameter of 150 nm.Other plasmonic coatings that can be used in accordance with the presentinvention include structures composed of gold, copper, platinum, alloysthereof, or other material displaying a strong plasmonic response at thewavelength of emission. The structure can be a random or a layer ofspheres, hemispheres, bow-ties, cylinders, or squares arranged as atwo-dimensional array. A similar structure that is also in accordancewith the present invention is a two-dimensional array of holes in a thinfilm where the holes are shaped as e.g., circles, ellipses, squares,rectangles, triangles, or bow ties.

Thus, in accordance with the present invention, a plasmonic layer metalnanoparticles can be deposited on the top n-type contact layer of aninverted LED structure, and if the n-type contact layer is sufficientlythin, surface plasmons from the metal layer can couple with the excitonsin the active QW emission region to provide an alternate decay route forthe excitons and improve the probability that the exciton will decay andscatter as photons emitted from the LED.

RESULTS

A two-dimensional coupled dipole approximation method was employed tomodel the electromagnetic behavior of an array of silver nanoparticleson the surface of AlGaN surrounded by air. See e.g., LinLin Zhao, K,Lance Kelly, and George C. Schatz, “The Extinction Spectra of SilverNanoparticle Arrays: Influence of Array Structure on Plasmon ResonanceWavelength and Width,” J. Phys. Chem. B 2003, 107, 7343-7350; Renebullock, Stefan Grafström, Paul R. Evans, Robert J. Pollard, and LukasM. Eng, “Metallic nanorod arrays: negative refraction and opticalproperties explained by retarded dipolar interactions.” J. Opt. Soc. Am.B, Vol. 27, No. 9 pp. 1819-1827 (September 2010).

FIG. 4 shows that alloying AN into the GaN contact layer leads to ablue-shift in the extinction peak. In the exemplary case illustrated inFIG. 4, Al_(0.1)Ga_(0.9)N was selected to improve injection efficiencyinto the GaN quantum well. FIG. 4 shows that a peak in extinctionefficiency was found at 365 nm for the array of silver nanoparticleswith a diameter of 150 nm and particle-to-particle center spacing of 225nm situated at an Al_(0.1)Ga_(0.9)N/air interface. The rapid thermalannealing conditions were selected to produce a set of silvernanoparticles that closely resembled this geometry, and thus match thesurface plasmon resonance to the quantum well near bandedge emission.

In addition, the model showed a complex dependence on inter-particlecoupling relative to the single particle Mie resonance. Confinement ofelectrons in small noble-metal nanoparticles leads to an electromagneticresonance upon plane-wave excitation. The charge within the silvernanoparticles move in phase, which creates an effective restoring forceat the particle dipole plasmon frequency. See Kullock et al., supra.Thus, leading to a corresponding dipolar field forming in the nearvicinity of the nanoparticles. Still, the electron-hole pairs in themetal can couple to the phonon bath, which leads to Ohmic heating. Inessence the imaginary component of the metal dielectric response limitsthe polarizability.

The luminescence of the sample was excited at 5K with the 325-nm line ofa He—Cd laser is illustrated in FIGS. 5A and 5B, which plot thephotoluminescence acquired at two different spectral ranges of aninverted LED with a silver nanoparticle coating. Two characteristicsample sets (plots 501/503 and 502/504, respectively) are displayed. Thesilver nanoparticles were designed to have a plasmon resonancecoinciding with the 365 nm near bandedge emission of the GaN quantumwell. The coupling with the on-resonant 365 nm emission led to anaverage approximate four times increase in luminescence.

The uncorrected photoluminescence spectra showed a factor of 5.2 and 3.6increase (on two separate samples) in near bandedge emission from thesilver coated structure in comparison to the emission prior todeposition. The wavelength of this enhancement (˜365 nm) coincided withthe surface plasmon resonance condition.

A time-resolved photoluminescence measurement found that the addition ofthe silver nanoparticles increased the photoluminescence decay rate by afactor of 3.81. As noted above, excitons generated in the active regioncan decay radiatively as a photon or non-radiatively as dissipated heatin the lattice; alternatively, the exciton energy can be transferred tothe surface plasmons. At this point, the coupled surface plasmon energyin the metal can decay as dissipated heat, which in turn would quenchthe emission. The increase in decay rate indicates that an alternatemechanism is provided by the nanostructure of the particles, whichcreates an ensemble of scattering sites that bridges the momentummismatch to rapidly convert the energy to free space radiation. See K.W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, LiWang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of topemission from ZnO thin film by nanopatterned Pt,” Applied PhysicsLetters 94, 151102 (2009). The similar increase in the intensity and thedecay ratio of the photoluminescence confirms that the spontaneousemission wavelength-dependent enhancement was clearly strongest at thesurface plasmon resonance energy where the surface plasmon DOS is high.See Liu, supra; see also Michael A. Mastro, Jaime A. Freitas Jr., OrestGlembocki, Charles R. Eddy Jr., R. T. Holm, Rich L. Henry, JoshCaldwell, Ronald W. Rendell, Fritz Kub and J. Kim, “Plasmonicallyenhanced emission from a group-III nitride nanowire emitter,”Nanotechnology 18 (2007) 265401.

CONCLUSION

Silver thin films and nanoparticles deposited on the surface of InGaNquantum well structures have been demonstrated to plasmonically enhancethe optically pumped emission from such structures. However, thenecessity of placing the active region of the device withinapproximately 30 nm of the plasmonic metal presents a design dilemma forGaN-based LEDs, and consequently, nearly all previous demonstrations ofexciton-to-surface plasmon coupling have been based on quantum wellstructures that are not designed for electroluminescence.

The present invention overcomes these drawbacks and provides plasmonicenhancement of an inverted LED structure. A variety of inverted LEDstructures were developed with the key design of a thin n-type(In,Al)GaN top contact layer which brings the quantum well into thefringing field of the silver nanoparticles. This proximity allowed theexcitons induced within the quantum well to couple to the surfaceplasmons, thereby increasing the emission by approximately a factor offour in a functional LED. Furthermore, the inverted LED allowsflexibility in the alloy composition of the top contact layer, whichalters the dielectric environment and plasmonic response of the metal.The approach of the present invention can be extended to any inverted(In,Al)GaN LED by tailoring the size of the noble metal nanoparticlesand consequently the location of the resonant surface plasmon energy tomatch the UV, violet, blue, or green emission.

In addition, the coupling concept of the present invention can beapplied to any plasmonic structure including:

A thick film similar to a reflective coating in which a suitableroughness will scatter the plasmon into photon;

A thin film that will have a surface plasmon at the semiconductor/metaland at the metal/air (or packaging material) interfaces, wherein acertain roughness can similarly scatter the plasmon into photon andwherein top and bottom surface plasmon polaritons can couple if themetal film is thin (i.e., <5 nm) forming a symmetric mode and a lossyasymmetric mode; and

A metal photonic crystal structure composed of an array of holes in athin film or the converse structure of an array of cylinders, squares,or similar structure, wherein the periodicity of the array is set todiffract light out of the structure.

Advantages and New Features

The coupling of plasmonic metal nanoparticles on the surface of astandard thick p-type (InAl)GaN contact layer to the underlying activeregion has been shown to be poor. Inverting the LED structure with athin n-type top layer brings the active QW emission region within thefringing field of the surface plasmonic metal and allows for coupling ofthe surface plasmons and the excitons from the active QW region which inturn provides enhanced emission from the LED.

Alternatives

The plasmonically enhanced light emitting device in accordance with thepresent invention can be composed of any semiconductor includingGaN/AlGaN/InGaN/InAlGaN, ZnO/MgZnO, ZnSe/ZnMgSe, CdTe/CdZnTe and similarsemiconductors.

In some embodiments, a second electrically injected light emitting diodeor laser diode can optically pump the primary light emitting device withthe plasmonic metal coating, with one or both devices stimulating aluminophoric medium in the same assembly.

The light emitting structure can also be shaped as one or more rods, orone or more wires.

The structure can also constitute a single component in amultiple-component module.

In any case, although particular embodiments, aspects, and features havebeen described and illustrated, one skilled in the art would readilyappreciate that the invention described herein is not limited to onlythose embodiments, aspects, and features but also contemplates any andall modifications within the spirit and scope of the underlyinginvention described and claimed herein, and such combinations andembodiments are within the scope of the present disclosure.

What is claimed is:
 1. A light emitting device having plasmonicallyenhanced emission, comprising: an inverted light emitting diode (LED)having a plasmonic metal coating thereon, the inverted LED comprising:an n-type substrate contacted to a first n-type electrode configured toprovide an Ohmic contact to the n-type substrate; an n-type cap layerdisposed on the n-type substrate; a tunnel junction disposed on then-type cap layer, the tunnel junction comprising a highly doped n-typelayer disposed on the n-type cap layer and a highly doped p-type layerdisposed on the highly doped n-type layer, the tunnel junction beingconfigured to allow a non-rectified current to flow from the n-typesubstrate into the n-type cap layer; and an active quantum well regiondisposed between a p-type layer on a bottom side thereof adjacent thep-type layer of the tunnel junction and an n-type contact layer on a topside thereof, the n-type contact layer having a plasmonic metal coatingcomprising a plurality of metal nanoparticles disposed on a firstdiscrete region of the n-type contact layer, the n-type contact layeralso having a second n-type electrode disposed on a second discreteregion of the n-type contact layer, the second n-type electrode beingcontacted to the n-type contact layer to provide an Ohmic contact to then-type contact layer, and the plasmonic metal coating and the secondn-type electrode forming separate and discrete structures on the n-typecontact layer; wherein the plasmonic metal coating is configured tomatch a plasmonic response to a peak emission from the active quantumwell region and to generate a plurality of surface plasmons at aninterface of the plasmonic metal coating and the n-type contact layer,the n-type contact layer being sufficiently thin to cause the activequantum well region to be within a fringing field of the surfaceplasmons; wherein electrons from the n-type contact layer and holes fromp-type layer form electron-hole pairs in the active quantum well regionupon the application of current to the inverted LED via the first andsecond n-type Ohmic contacts; wherein a first plurality of theelectron-hole pairs decay via a first decay route to produce a firstlight emission from the light-emitting device; and wherein the secondplurality of the electron-hole pairs couple with a correspondingplurality of the surface plasmons and decay via a second decay routeprovided by the coupling of the electron-hole pairs and the surfaceplasmons to produce a second light emission from the light-emittingdevice; the first and second light emissions providing an enhanced lightemission from the light-emitting device.
 2. The light emitting deviceaccording to claim 1, wherein the n-type contact layer has a thicknessof less than 50 nm.
 3. The light emitting device according to claim 1,wherein the n-type contact layer has a thickness of less than 30 nm. 4.The light emitting device according to claim 1, wherein the n-typecontact layer comprises one of GaN, AlGaN, InGaN, InAlGaN.
 5. The lightemitting device according to claim 1, wherein the n-type contact layeris a highly doped n+ AlGaN layer having a thickness of about 20 nm. 6.The light emitting device according to claim 1, wherein the p-type layercomprises one of GaN, AlGaN, InGaN, InAlGaN.
 7. The light emittingdevice according to claim 1, wherein the p-type layer has a thickness ofabout 10 to 1000 nm.
 8. The light emitting device according to claim 1,wherein the p-type layer is a highly doped p+ GaN layer having athickness of about 500 nm.
 9. The light emitting device according toclaim 1, wherein the tunnel junction comprises a p++ GaN layer disposedon an n++ GaN layer.
 10. The light emitting device according to claim 9,wherein each of the p++ GaN layer and the n++ GaN layer have a thicknessof 5 nm.
 11. The light emitting device according to claim 1, wherein then-type cap layer is an n-type GaN layer.
 12. The light emitting deviceaccording to claim 1, wherein the plasmonic metal coating comprises aperiodic coating of metal nanoparticles.
 13. The light emitting deviceaccording to claim 1, wherein the plasmonic metal coating comprises acoating of one of silver, gold, and copper.